One means of molding thermosetting polymeric articles is the injection of a chemically reactive liquid precursor into a mold. Once in the mold, a rapid polymerization reaction takes place yielding a hardened, chemically cross-linked polymeric article. This process is generally referred to as reaction injection molding (RIM).
In the RIM process, it is extremely undesirable to have the reactive constituents polymerize and harden in the injection equipment prior to the entry into the mold. In order to achieve good mold cycle times, however, it is imperative that the liquid precursor set up within seconds of mold injection. These two ends tend to be in conflict with one another.
At this time, RIM systems are used to mold large polyurethane articles such as automotive body panels and fascia. Polyurethane is formed by the rapid polymerization of a mixture of highly catalyzed polyol and isocyanate constituents in a mold. The polyol and isocyanate constituents are fairly inert individually, but react very rapidly once they are mixed. Accordingly, the components are initially retained in separate containers and impingement mixed as needed immediately prior to molding. In some systems, the reactive mixture is momentarily accumulated in a cylindrical injector barrel and a plunger is used to rapidly eject the mixture into the mold. The accumulator is generally provided with cooling means to retard polymerization. For the same reason, the accumulator barrel inlet is generally located some distance from the heated mold.
A problem with this type of RIM system is that if the injection plunger is stroked fully forward to empty the accumulator barrel on each shot, a vacuum is created as the plunger is retracted which causes air to be drawn into the accumulator. The air mixes with incoming liquid constituents creating bubbles and voids in the next article molded. This problem may be avoided by never allowing the plunger to move forward of the barrel inlet port for the reactive liquid constituents. However, an excessive amount of reactive material then remains in the accumulator barrel between molding cycles. Even with cooling, the constituents have a tendency to begin to set-up which ruins or reduces the quality of subsequently molded parts.
Thus, means have been sought to provide for rapid delivery of RIM constituents from an accumulator to a mold such that reciprocation of the plunger causes only a minimal amount of reactive material to remain in the accumulator barrel between mold cycles and so that little or no air is introduced.